1. Field of the Invention
This invention relates to a sheet punch device which includes a punch member to punch holes in a sheet member.
2. Discussion of the Background
A known sheet punch device, which is installed in an image forming apparatus such as a printer, a photocopying machine, a facsimile, or the like or in a sheet post-processing apparatus connected with the image forming apparatus, includes a punch member to punch holes in a sheet member, such as a sheet of paper (hereinafter referred to as a sheet). The sheet post-processing apparatus performs several operations for a sheet discharged from the image forming apparatus. The operations include, for example, a stack operation for stacking sheets on a sheet discharging tray with the sheets divided by page number, a staple operation for stapling a set of sheets and discharging the stapled sheets on the sheet discharging tray, and a punching operation for punching a sheet or sheets with the sheet punch device. Background sheet punch devices are divided into two types depending on the driving method for driving a punch and a die to punch a hole in a sheet, i.e., (1) a rotary punch method and (2) a press punch method.
As sheet punch devices of the rotary punch method, there are, for example, two types depending on the driving mechanism. In the first type of sheet punch device, a punch and a die are driven to rotate via a clutch by a motor to punch a hole in a sheet. The same motor also drives a pair of sheet conveying rollers to convey a sheet. The second type of sheet punch device includes a sheet conveying motor and another motor which drives and rotates a punch and a die mounted on a sheet conveying path, as described in Japanese Laid-open Patent Publication No. 7-136995 of 1995. In JP No. 7-136995, it is described that the second type of the sheet punch device can more precisely punch a hole at a predetermined position in a sheet than the first type of the sheet punch device, because the punch and the die are driven by a separate motor from the motor driving a pair of sheet conveying rollers to convey a sheet.
Japanese Laid-open Patent Publications No. 9-136762 (1997) and No. 9-249348 (1997) describe a sheet punch device of the press punch method, in which a hole is punched in a sheet by driving a punch and a die by a solenoid or a motor to move the punch forward and backward rectilinearly toward the die which is placed at a predetermined position.
While the sheet punch device of the press punch method punches a hole in a sheet with the sheet stopped at a certain position, the sheet punch device of the rotary punch method punches a hole in a sheet without stopping the sheet. The sheet punch device of the rotary punch method can punch a hole in an advancing sheet with a simple mechanism of rotating a punch and a die in the same direction as the sheet advancing direction. Therefore, the rotary punch method has advantages in productivity, simple mechanism, and thereby in low costs.
Another sheet punch device of the rotary punch method is proposed in Japanese Laid-open Patent Publication No. 6-278095 (1994), in which two or three punch and die pairs, which are disposed in the sheet width direction perpendicular to the sheet conveying direction, rotate synchronously with each other and punch two or three holes simultaneously at predetermined positions in a copy sheet fed from a photocopying machine (e.g., image forming apparatus).
However, the functions of the above-described background rotary sheet punch devices are limited. For example, the background sheet punch devices can only punch holes at plural positions in a sheet in the sheet width direction perpendicular to the sheet conveying direction, but cannot punch holes at plural positions in the sheet conveying direction.
Moreover, in the sheet punch device of the rotary punch method described in JP No. 6-278095, the punch and die pairs are fixed at the positions spaced a predetermined distance apart in the sheet width direction, so that the number and position of holes punched in a sheet are determined by the number of punch and die pairs in the sheet punch device. Furthermore, because the plurality of punch and die pairs perform punching operations in a sheet simultaneously, the load on each driving shaft of the punches and the dies increases. The load transmits a driving force to the punches and dies to drive them. Therefore, when the punch and die pairs are used to punch holes simultaneously, the power of a driving motor, which drives and rotates the plurality of punch and die pairs, needs to be increased.